Thermodynamics, isotherms, and mechanisms studies of lithium recovery from seawater desalination reverse osmosis brine using roasted and ferrocyanide modified date pits

Abstract

In this study, the adsorption isotherms and thermodynamic studies of lithium ions from seawater reverse osmosis (SWRO) desalination brine were investigated. Three adsorbents were utilized namely, roasted date pits (RDP), potassium copper, and nickel hexacyanoferrate-date pits (RDP-FC-Cu and RDP-FC-Ni). The prepared adsorbents showed enhanced morphological and chemical structures such as high porosity, carbonaceous composition, larger pore and volume sizes, smaller particle sizes as well as the presence of unique functional groups on their surface. The adsorption of lithium ions onto the three adsorbents was enhanced with an increase in solution temperature and initial lithium concentration. The temperature that showed the highest adsorption of lithium ions onto the three adsorbents was 45 °C. The adsorption of lithium ions onto the three adsorbents was the highest at an initial lithium concentration of 100 mg/L. The three adsorbents achieved an adsorption capacity of around 99 mg/g at the optimum temperature and initial concentration. On the other hand, RDP-FC-Cu achieved the highest adsorption capacities for lithium ions at all the studied initial concentrations. The thermodynamic study showed that the adsorption process of lithium ions onto the adsorbents is endothermic, spontaneous, and favorable at all the studied temperatures (25 °C, 35 °C, and 45 °C). Moreover, the adsorption of lithium ions onto the three adsorbents followed the Langmuir, Freundlich, Dubinin–Radushkevich, and Temkin adsorption isotherm models differently at each studied temperature. For RDP, the adsorption process followed the Freundlich adsorption isotherm model at 25 °C, while it was more fitted to the Langmuir isotherm model at 45 °C and all models at 35 °C. The adsorption of lithium ions onto RDP-FC-Cu followed Langmuir adsorption isotherm model at 25 °C and 35 °C, while it fitted all models at 45 °C. On the other hand, Langmuir and Dubinin–Radushkevich isotherm models were best fit for the adsorption of lithium ions onto RDP-FC-Ni at 25 °C and 35 °C. The desorption study presented 99% desorption percentages of lithium ions from all the adsorbents, which showed the great regeneration potential of the adsorbents. Furthermore, the selectivity study showed that RDP-FC-Cu achieved 99.9% adsorption removal of lithium ions from the SWRO brine while RDP-FC-Ni and RDP achieved 99.8% and 99.3% adsorption removals, respectively. Finally, the cost analysis revealed that the total cost for the preparation of the adsorbent was 29.81 USD.